1. Field of the Invention
The present invention relates to a data modulation and demodulation system useful in a magnetic recording system. More particularly, it relates to a run length limited (RLL) type data modulation and demodulation system for a magnetic recording system.
In a magnetic recording system, such as a magnetic disc system, write data is modulated under synchronization with a system clock signal and the modulated data is stored in a magnetic disc. Conversely, the modulated and stored data is read and demodulated to restore original data used outside of the magnetic disc system. A data modulation and demodulation system is employed to achieve the above data encoding and decoding.
2. Description of the Related Art
A variety of modulation methods have been proposed for increasing the recorded density of data on magnetic media such as a magnetic disc. Recently, a modulation method, known as "variable length constant rate coding" and using a run length limited (RLL) format, has been applied to the magnetic recording system.
The variable length constant rate coding is carried out various ways, such as a coding of a (1,7) or 1/7 format with a rate of 2/3, hereinafter 1/7 coding, a coding of (2,7) or 2/7 format with a rate of 1/2, hereinafter 2/7 coding, and a coding of (1,8) or a 1/8 format. The 1/7 coding converts data with a rate of 2/3: two bits to three bits, and inserts a zero ("0"), up to seven zero bits, between consecutive one ("1") bits data. The 2/7 coding converts data with a rate of 1/2: one bit to two bits, and inserts at least two series zero ("0") bits, up to seven zero bits, between consecutive one ("1") bit data. Compared with the 1/7 coding and the 2/7 coding, the 1/7 coding does not require a high speed operable circuit and has a wide data detection window, but has a disadvantage of a low recording density. Conversely, the 2/7 coding achieves a high recording density, but requires a high speed operation circuit.
U.S. Pat. No. 4,488,142 discloses a (1,7) format for encoding and decoding with a rate of 2/3. The present invention also uses the (1,7) format encoding and decoding with a rate of 2/3, i.e., the 1/7 coding. However, U.S. Pat. No. 4,488,142 does not disclose a data modulation and demodulation system in which the 1/7 coding is applied.